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release/19.x: [libc++] Increase atomic_ref's required alignment for small types (#99654) #101492

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Merged
tru merged 1 commit into from
Aug 2, 2024
Merged

release/19.x: [libc++] Increase atomic_ref's required alignment for small types (#99654) #101492

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17 changes: 11 additions & 6 deletions libcxx/include/__atomic/atomic_ref.h
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Original file line number Diff line number Diff line change
Expand Up @@ -57,11 +57,6 @@ struct __get_aligner_instance {

template <class _Tp>
struct __atomic_ref_base {
protected:
_Tp* __ptr_;

_LIBCPP_HIDE_FROM_ABI __atomic_ref_base(_Tp& __obj) : __ptr_(std::addressof(__obj)) {}

private:
_LIBCPP_HIDE_FROM_ABI static _Tp* __clear_padding(_Tp& __val) noexcept {
_Tp* __ptr = std::addressof(__val);
Expand Down Expand Up @@ -108,10 +103,14 @@ struct __atomic_ref_base {

friend struct __atomic_waitable_traits<__atomic_ref_base<_Tp>>;

// require types that are 1, 2, 4, 8, or 16 bytes in length to be aligned to at least their size to be potentially
// used lock-free
static constexpr size_t __min_alignment = (sizeof(_Tp) & (sizeof(_Tp) - 1)) || (sizeof(_Tp) > 16) ? 0 : sizeof(_Tp);

public:
using value_type = _Tp;

static constexpr size_t required_alignment = alignof(_Tp);
static constexpr size_t required_alignment = alignof(_Tp) > __min_alignment ? alignof(_Tp) : __min_alignment;

// The __atomic_always_lock_free builtin takes into account the alignment of the pointer if provided,
// so we create a fake pointer with a suitable alignment when querying it. Note that we are guaranteed
Expand Down Expand Up @@ -218,6 +217,12 @@ struct __atomic_ref_base {
}
_LIBCPP_HIDE_FROM_ABI void notify_one() const noexcept { std::__atomic_notify_one(*this); }
_LIBCPP_HIDE_FROM_ABI void notify_all() const noexcept { std::__atomic_notify_all(*this); }

protected:
typedef _Tp _Aligned_Tp __attribute__((aligned(required_alignment)));
_Aligned_Tp* __ptr_;

_LIBCPP_HIDE_FROM_ABI __atomic_ref_base(_Tp& __obj) : __ptr_(std::addressof(__obj)) {}
};

template <class _Tp>
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2 changes: 1 addition & 1 deletion libcxx/test/std/atomics/atomics.ref/is_always_lock_free.pass.cpp
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Expand Up @@ -54,7 +54,7 @@ void check_always_lock_free(std::atomic_ref<T> const& a) {
#define CHECK_ALWAYS_LOCK_FREE(T) \
do { \
typedef T type; \
type obj{}; \
alignas(std::atomic_ref<type>::required_alignment) type obj{}; \
std::atomic_ref<type> a(obj); \
check_always_lock_free(a); \
} while (0)
Expand Down